Emulsions consist of immiscible liquid phases stabilized by one or more surfactants. The need to ensure enhanced performance and to extend the fields of application of emulsions requires calibration of their particle size. In the case of emulsified bitumen for example, the improvement in the properties of the emulsion, in particular in the area of road surfacing (ease and safety of use, homogeneity after drying . . . ), necessitates the obtaining of a finer particle size than currently produced by industrial units. By finer particle size is meant a reduction in the mean size of the droplets and in their polydispersity compared with existing methods.
Two methods can be considered to modify the particle size of an emulsion:
1) a change in the physicochemical parameters of the emulsion,
2) a change in the manufacturing process, or emulsifying process.
However, the specific applications of emulsions often restrict modifications related to physicochemical parameters, which means that modification of the emulsifying process remains practically the only possible way to achieve this objective.
Emulsifying methods are generally developed and scaled under turbulence conditions. Prior art emulsification under these conditions led to identifying a size criterion, which relates the mean droplet size with the power dissipated in the mixer. Technological developments in emulsification methods have therefore turned towards maximizing and/or controlling the dissipated power in mixture geometries. Typically, locally dissipated power varies between 104 W/m3 and 107 W/m3 and the peripheral speed of the impeller is greater than 10 m/s. According to the above-described approach, the success of this objective to control and reduce particle size relies on the design of better performing equipment (high speed rotating parts on geometries provided with a gap of generally less than 1 mm). Said design generates major mechanical complications that are even greater on industrial units. Additionally, this intensification in dissipated power is often accompanied by a major decrease in the residence time in the shear zone, thereby aggravating phenomena of re-coalescence of the droplets and limiting the expected effect of dissipated power on the mean droplet diameter. This is why conventional emulsification methods available on an industrial scale remain largely unsatisfactory.
Also, it is to be noted that the production of high disperse phase emulsions (i.e. with an internal phase of more than around 70%) generally has recourse to specific techniques.
As an example of a method of emulsification in high concentration conditions, document GB 1283462 proposes a system for the continuous production of an oil-in-water emulsion, comprising a rotating beater of planetary type, and in which the phases to be emulsified and the formed emulsion are respectively added and withdrawn continuously.
Document U.S. Pat. No. 3,565,817 gives another example of a method for the continuous production of a concentrated emulsion, in which shearing must be maintained at a sufficient value to reduce the viscosity of the emulsion, but at less than the instability point of the emulsion.
Documents EP 0156486 and EP 0162591 describe methods for preparing concentrated emulsions, at a shear rate of between 10 and 1000 s−1, but which, in practice, only allow droplets to be obtained having a typical size of 2 μm to 50 μm.
Document U.S. Pat. No. 4,746,460 describes a method for preparing a concentrated emulsion produced from a foam obtained by beating an aqueous solution with a gas.
Document U.S. Pat. No. 5,250,576 describes a more particular application of a method for preparing concentrated emulsions in which the emulsion is stabilized by cross-linking polymers.
In document U.S. Pat. No. 5,399,293 a concentrated emulsion is continuously formed by subjecting the liquid to two separate, successive shear forces with a single shaft mixer. However, it appears in the examples that the system does not allow droplets of a size of less than 3 μm to be obtained.
Document U.S. Pat. No. 5,539,021 presents another method for preparing a concentrated emulsion, in which the important parameter is the adjustment of the respective flow rates of the two phases to be emulsified, which are continuously mixed.
Document U.S. Pat. No. 5,827,909 describes a continuous method for preparing an emulsion, in which part of the emulsion is withdrawn from the mixing area then re-injected into the mixing area. This method is more particularly dedicated to emulsions intended to undergo subsequent polymerization.
Document WO 99/06139 proposes mixing a first viscous phase to be emulsified (having a viscosity of between 1 and 5000 Pa·s) with a second phase non-miscible with the first one, at a proportion of 75 to 90 wt. % of first phase and a shear rate of between 250 and 2500 s−1. The method described in this document is discontinuous i.e. the two phases are brought together at one time.
However, the methods described in the above documents remain difficult to implement. In particular the concentrated emulsions have major instability problems and high risks of phase inversion (i.e. risks of changing from an emulsion of oil-in-water type to an emulsion of water-in-oil type; they also have specific problems related to their non-Newtonian, elastic rheological behaviour.
There is therefore a need to improve known methods, allowing to prepare emulsions in a more reliable and more reproducible manner, with a controlled (and the smallest possible) particle size in terms of mean droplet diameter and polydispersity, in particular on the scale of commercial or industrial production.